This invention relates to solid-state light sources and particularly to light emitting diodes (LEDs). More particularly, the present invention relates to an apparatus for controlling power and current delivered to the LED.
Light emitting diodes (LEDs) are semiconductor devices that generate light when electrical energy (e.g., current, voltage) is applied to the device. A driver (i.e., one or more electronic components electrically connected to an LED) may be used for selectively applying electrical energy from an electrical energy source to the LED. A conventional LED driver has a particular topology for use with the electrical energy source to provide constant current to the LED. Thus, the LED driver allows the LED to continuously operate at a constant current level. The topology for the driver may be buck, boost, or a combination buck and boost (hereinafter referred to as “boost buck”), and is selected based on the electrical energy available from the electrical energy source and the electrical characteristics of the LED (e.g., forward voltage). For example, an LED driver having a boost buck topology must be used to drive an LED wherein the forward voltage of the LED may overlap the available source voltage.
Not all of the electrical energy applied to the LED is converted to light. A substantial portion of the applied electrical energy is dissipated in the form of heat by the LED. As the semiconductor material heats up, like most electronics devices, the LED performance is degraded. In particular, the power (e.g., heat) dissipated by the LED can cause decreased light output (flux), a color shift, and a reduction in device lifetime. To minimize the adverse effects of the power dissipation, various thermal management systems may be incorporated or used in conjunction with the LED.
For an LED driven from a driver having a constant current boost buck topology the power dissipation of the LED and hence the driver can vary, depending on the forward voltage of the LED. Conventionally, a thermal management system which assumes the worst case power dissipation is employed to avoid an excessive temperature rise resulting from the variable power dissipation of the LED. Referring to FIG. 1, for example, the driver may provide a constant current (Iled) to the LED until the forward voltage (Vf) of the LED reaches a maximum value (Vled max). When this condition is met, the driver shuts down or enters into a fail safe mode (i.e., ceases to apply current to the LED). However, this type of thermal management system fails to effectively solve the problem. In particular, the system can cause oversized heat sinks and/or require active cooling techniques.
FIG. 2 illustrates another technique for minimizing the adverse effects of variable power dissipation used for applications other than driving LEDs. As illustrated, the driver provides electrical energy to the load such that the load operates at a constant power and for a particular range of current and voltage values. Thus, the driver may provide current less than a maximum value (Iled max), as a function of the constant power value, until the voltage of the load reaches a maximum value (Vled max). When this condition is met, the driver shuts down or enters into a fail safe mode (i.e., ceases to apply current to the load). On the other hand, as the forward voltage decreases, to maintain a constant power level, the controller increases the operating current. Since the operating current needs to be limited to protect the LEDs, the driver needs to shut down upon reaching the maximum current, Iled max, which limits the operating region of the system